This may come as a surprise to some of you, but we theater critics do not love to bash a show. It gives us no pleasure to pan a poor production, no matter how much it deserves it, that fades in view even while we're watching it. However deserving of a sharp pen a show might be, we don't like to use it. What we want to do is praise. We want to shout our joy when theater comes alive, moves us and makes us rethink whatever we believe we know about the theater. We ache, in a good way, to see a show that comes out of the blue and shocks us with the magic of live performance, that exhilarates, that leaves us breathless and spellbound. We want to send others to it so that they, too, can bask in the wonders that only great theater can accomplish -- a piece of theater that leaves us cleansed and renewed.

That kind of show, forever rare and out of reach, is fervently on display at the Alley. It is the Cornish-based Kneehigh's stunning production of Tristan & Yseult (2004), currently on a world tour. We mightily thank the Alley for bringing this radiant work of art to us. Without question, this is the best show of the season. It will thrill you, make you laugh, make you weep and send you home on clouds of unbridled inebriation. This is what theater strives for and hardly ever attains. Let's begin the praise; no, the hosannas!

The wild and woolly county of Cornwall, that spit of a peninsula that juts southwest out of England, is home to the legendary Celtic Brythons, equally wild and woolly. It's a landscape of rugged cliffs and heathered romantic moors, with jagged coastline fit for pirates and shipwrecks, as in Hitchcock's Jamaica Inn or as the comic base for Gilbert & Sullivan's Pirates of Penzance. Cornwall is home to Maxim de Winter's Manderley in DuMaurier's haunting Rebecca, and it's the land where the woeful medieval tragedy of besotted Tristan and Yseult probably began, since the Irish princess in the fable, in love with Tristan owing to an unnecessary love potion, is brought to the Cornish court to wed King Mark.

Cornwall is also the apt home base of Kneehigh, one of the world's most inventive theater companies. Renowned for its provocative and endlessly imaginative productions (The Red Shoes, The Bacchae, Don John, Rebecca, Dead Dog in a Suitcase), the company adores site-specific sites -- cliff faces, ruined castles, back alleyways -- and uses every available device known to showbiz in its expressive shows. Nothing is off-limits so long as it works. T&Y works like gangbusters. It's a most fantastic mash-up of Monty Python, Cirque du Soleil, Shakespearean high drama, English panto, song-and-dance, samplings of Wagner's great opera, a handmaiden in drag, lilting folk ballads. But it never once loses focus on its overarching theme of love: love found, love lost, love betrayed, love dried up or love never found at all. The impertinent pairing of high and low is some sort of genius.

We're in the retro Club of the Unloved, where appropriately sad pop songs like Roy Orbison's "Only the Lonely" or Bobby Darren's "Dream Lover" are crooned by the upstage band. Called the Lovespotters and the Kiss Clockers, the unloved yearn for their membership to expire, so they scan the audience through binoculars. They're only looking for love. They wear windbreakers, balaclavas and sunglasses. Whenever someone joins the club, they don the uniform. A mysterious woman in pillbox hat, white gloves and '50s couture, called Whitehands (Kristy Woodward), sets the story in motion, but not before shouting, "Why don't you love me?!" to no one in particular. By the end, her question will be answered with heartaching clarity.

In the tale, spun so rhapsodically by Kneehigh's creators Carl Grose, Anna Maria Murphy and director/adapter Emma Rice, the kingdom of King Mark (Stuart Goodwin) is besieged by rapacious Morholt of Ireland (Niall Ashdown). To the rescue comes photogenic French knight Tristan (Dominic Marsh), "born in sorrow," whose misadventures have brought him to the wild shores of Cornwall. Morholt is savagely dispatched by him in an action-film brawl, after which Morholt's sister Yseult (Hannah Vassallo), whose single strand of hair has inflamed the loveless King, is a ripe target as one of the spoils of war.

Sent to Ireland to fetch her and bring her to the anxious king, the wounded Tristan lies in a hammock. Stripped of her "unloved costume" that reveals her in pink sundress, the fair Yseult warily approaches, rolls underneath the swinging hammock and gracefully joins him in the swing. "My fate is in my hands," she purrs, straddling the young warrior, as she touches his wound and heals him. It's definitely love at first sight in this detailed choreographed seduction. But her passion turns to hate when she discovers that he has killed her brother. That's when the "love potion" comes into play, brought forth by comic handmaiden Brangian (Niall Ashdown in babushka scarf and tennis shoes). The hot young couple falls into delirious love, swinging wildly on ropes controlled by the Lovespotters, who fling them around the stage like dizzy gymnasts. They kiss, fly, embrace, fly, fondle, fly, then kiss again in spirited abandon.

The story deepens and spins richly as drunken passion overwhelms them. Mark's toady Frocin (Damon Daunno), jealous that he has lost favor with the king when shining Tristan eclipsed his influence, is hell-bent on revenge, itching to expose the lovers. Lovelorn Brangian, misused by her mistress, substitutes for her on the wedding night, slipping into the king's bed. So blinded by love he sports a blindfold, Mark isn't aware of the ruse. When the lovers run off for a tryst, they are discovered by Frocin, whose incriminating Polaroid breaks the heart of the betrayed king. The tragedy spirals to a shattering conclusion, powerfully scored to Richard Wagner's climactic and searing "Love/Death." The billowing white sail of Yseult's ship fills the stage, as she races to save her dying lover.

Love heals, love scars, love is comic, love is grand. As the wise men say, Love is all. T&Y lays it all out with innumerable coups de théâtres, wondrous scenic effects and bursts of stage wizardry. Top to bottom, the entire cast, whether singing, dancing, mugging or emoting, is irrepressible. Their joy in performing is infectious. Róbert Luckay and Gareth Charlton, playing multiple subsidiary roles as Lovespotters, Brutes and Animators, round out the impressive cast. The marvelous, evocative score by Stu Barker, with its tinge of Klezmer and baleful Irish whine, using clarinet, mandolin, ukulele, cello drone and dulcimer (the dulcimer gets its own solo comic riff), is spiritedly played by Pat Moran, James Gow, Justin Radford and Lizzy Westcott, and sung at times by all the actors. Then there's designer Bill Mitchell's simple set of circular stages and second-story gangplank with exit sign lighting the way for the unloved. Or lighting designer Malcolm Rippeth's crisp atmospheric overlay and sound designer Gregory Clarke's percussive thumps, whaps and thuds. It's all grand, put into motion by director Emma Rice, whose unfailing eye and sense of timing are uncanny.

The ancient tale of Tristan & Yseult, spectacularly brought up to date by Kneehigh, speaks to all of us. The show resonates and reverberates, and is thoroughly affecting. It transports us.

If you've seen Mary Zimmerman's sparkling Metamorphoses or Rick Elice's delightful vaudeville Peter and the Starcatcher, you know the redemptive power of theater at its most magical. T&Y traffics in love, its tragic highs and comic lows, but at its heart it's about love of theater and the simple sorcery of stage imagination and how completely it can conjure the depths of the human spirit. This glorious play is one of a kind, grandly inventive and immensely moving, and I urge you to experience it. Bold, brave and thoroughly alive, Tristan & Yseult will not change your life or how you love or even help you find love, but you will never forget it. And that is true theater magic at its most potent.